A "trap" is a method of overlapping abutting colored objects to compensate
for the imperfect registration of printing presses. Because registration, even
on good presses with good operators, can be off by a quarter point or more,abutting
elements in your publication may not end up abutting perfectly when the publication
is printed by your commercial printer. What happens then? The paper shows through
where you don't want it to (see Color Example 2 and Example 3).

Do I need to tell you what happens when you take your work to a press that's
badly out of register or run by turkeys? Disaster. Before this happens to you,
talk with your commercial printer regarding the tolerances of their presses
and/or operators. Don't ask them if they're turkeys- it's considered rude.

Manual Trapping

If you can't (or don't want to) use InDesign's automatic trapping methods (In-RIP
or built-in),you can still trap your publication - you'll just have to do it
yourself. I'll describe the process, because I believe that you should know
how to add and subtract, multiply and divide before you ever use a calculator.

Before I start describing manual trapping techniques, however, I need to state
that InDesign's automatic trapping methods can trap your publications better
than you can (assuming that you have both deadlines and a finite amount of patience),
and if you use them, you usually won't even have to think about trapping.

Object-Level Overprinting. The key to trapping,in InDesign and elsewhere,is
in control ling which objects - or which parts of objects - print on top of
other objects as the printing press prints your publication. The only way to
make manual trapping work is to control the overprinting characteristics of
individual objects (see Color Example
1 and Example 4).

Luckily, you can. Any InDesign path can be specified as an over-printing object
(that is, it won't knock a hole in any objects behind it when you print), regardless
of the object's color. The controls for object-level overprinting are the Overprint
Fill and Overprint Stroke options found in the Attributes palette (see Figure
1). These controls, used in combination with InDesign's Paste Into command,
can be used to create virtually any trap.

I have to stress the importance of the Weight Changes Bounding Box option
on the Stroke palette menu. You cannot create traps when this option is turned
off,so you 'l l have to make sure it's turned on as you follow any of the procedures
in this section.

When you're working with trapping, you'll be creating spreads (outlines of
objects, in the same color as the object,that are slightly larger than the object
itself) and chokes (outlines of the object that are the same color as the underlying
object's color). Spreads make the object larger so that the edges of the object
print over the underlying object; chokes make the area knocked out of the underlying
object smaller than the outline of the foreground object.

Use chokes when the foreground object is a darker color than the background
object; use spreads when the foreground object is lighter. In other words, trap
from light colors into darker colors. Sound subjective? It is. I use chokes
when I'm trapping type-text characters of ten look distorted when you use spreads
(the eye is very critical when it comes to text).